Solar cells can be connected in series to form a photovoltaic (“PV”) string. The PV string can include several solar cells and/or PV modules, with each PV module comprising solar cells mounted on the same frame. In a PV string, the positive terminal of one solar cell is electrically connected to the negative terminal of another solar cell, the positive terminal of the other solar cell is electrically connective to the negative terminal of yet another solar cell, and so on. The output voltage of a PV string depends on the number of solar cells in the string.
In a series-connected string of direct current (“DC”) power sources, the voltage of each source adds to produce the voltage delivered to a load. In solar energy systems, solar modules, also referred to herein as PV modules, are typically connected in series to provide voltage to the load (e.g., an inverter, a battery system). Each PV module can include one or more cell strings coupled in series with each cell string including one or more solar cells coupled in series. Typically, each string of PV modules is connected to an inverter, or multiple strings of PV modules are coupled in parallel at a combiner box, and one or more combiner boxes lead to a central inverter. In some instances, a system may have a maximum voltage limit, for example, due to hardware (e.g., inverter) limitations, safety, and/or reliability reasons. Accordingly, the number of PV modules and/or cell strings that can be connected in series can be limited by that maximum voltage.
The maximum voltage produced by a series of solar cells can occur when the system is producing no current and each solar cell operates at its open-circuit voltage (VOC). This condition can occur due to an inverter shutdown, system damage, or during transient measurements taken by the inverter to understand system performance, among other reasons. Systems must be designed to accommodate the maximum voltage condition even though it often does not occur. Additionally, the maximum voltage produced by solar cells increases at low temperatures, leading to an additional derate factor that limits the number of series-connected solar cells. As a result, systems must be designed with a voltage buffer to accommodate VOC. Inefficiencies resulting from the overdesign include fewer modules per string of modules, oversized wiring, among other inefficiencies, which lead to a higher balance of system (BOS) cost.
Moreover, in some instances, one or more of the cells in a string can underperform due to soiling, shading, defects, and/or damage, which can limit the performance of a cell string and/or module. If the current of the cell is reduced enough, the cell can operate in reverse bias and may even reach reverse breakdown. A cell in reverse bias can cause hot spots, which can severely damage the PV module.